1. Field of the Invention
The present invention relates in general to an improved digital storage system and in particular to a method and system for improving the bandwidth of a voice coil actuator (VCA) within a hard disk drive (HDD). More particularly, the present invention relates to an improved method and system for compensating for second-order resonant frequency effects within a VCA coil and carriage assembly. Still more particularly, the present invention relates to detecting and tracking butterfly modal parameters within a VCA, such that butterfly mode frequency and amplitude may be anticipated, and undertaking remedial compensation measures in response thereto, such that the contribution of mechanical deformation interference with VCA head positioning is minimized.
2. Description of the Related Art
Generally, a digital data storage system consists of one or more storage devices that store data on storage media such as magnetic or optical data storage disks. In magnetic disk storage systems, a storage device is called a hard disk drive (HDD), and includes one or more hard disks and an HDD controller to manage local operations concerning the disks. Hard disks are rigid platters, typically made of aluminum alloy or a mixture of glass and ceramic, covered with a magnetic coating. Typically, two or three platters are stacked vertically on a common spindle that is turned by a disk drive motor at speeds often exceeding ten thousand revolutions per minute (rpm).
The only other moving part within a typical HDD is the head assembly. Within most drives, one read/write head is associated with each side of each platter and flies just above or below the platter""s surface. Each read/write head is connected to a flexible arm apparatus which supports the entire head flying unit. More than one of such arms may be utilized together to form a single armature unit.
Each read/write head scans the hard disk platter surface during a xe2x80x9creadxe2x80x9d or xe2x80x9cwritexe2x80x9d operation. The head/arm assembly is moved utilizing an actuator which is often a VCA driven by a servo voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which is mounted a spindle supporting the disks. The base casting is in turn mounted to a frame via a compliant suspension. When current is fed to the motor, the VCM develops force or torque which is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head nears the desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop directly over the desired track.
Modern HDD throughput and storage capacity have been substantially enhanced by improvement in actuator design which has resulted in increased precision and speed in head placement. The more precisely the actuator can place the read/write head, the greater the amount of data that can be packed onto a given area of disk surface (often referred to as areal density). The term xe2x80x9cservo bandwidthxe2x80x9d will be utilized hereinafter to denote the cross-over frequency of an open loop transfer function (See Appendix A) applied to a head positioning system. The demand for increased speed and storage capacity has resulted in ever faster and more compact hard disk drive (HDD) assemblies. As the track densities of HDDs increase, a high servo bandwidth is required to improve the Track Misregistration (TMR) performance. Mechanical resonance of the coil and carriage is one of the dominant factors that limit the servo bandwidth of a voice coil driven HDD. As the track density of HDDs increases, a high servo bandwidth is required to improve the efficiency of read/write operations as measured by TMR as well as other performance indicators.
It would therefore be desirable to provide an improved method and system for minimizing the contribution of dynamic mechanical deformation of a HDD suspension and head apparatus to the off-track position error of read/write heads in a digital recording system. If implemented, such a system would serve to increase the servo bandwidth and thus the effective track density of a HDD assembly.
It is therefore an object of the invention to provide a method and system for improving the bandwidth of a voice coil actuator (VCA) within a hard disk drive (HDD).
It is another object of the invention to provide an improved method and system for adaptively compensating for resonant frequency effects within a VCA coil and carriage assembly.
It is still another object of the invention to provide an improved method and system for anticipating real-time resonant vibration characteristics and undertaking remedial compensation measures in response thereto, such that the contribution of mechanical deformation to interference with VCA head positioning is minimized.
The above and other objects are achieved as is now described. A system and method for adaptively compensating for real-time variations in mechanical dynamics of a head-positioning assembly during track follow and seek operations are disclosed. The head-positioning assembly includes a voice coil actuator that positions a read/write head utilizing a coil and carriage in conformity with an actuator control signal. Predetermined temperature dependent calibration measurements are utilized to predict resonant mode characteristics, such as amplitude and frequency of butterfly mode resonance, at different coil temperatures. Variations in these butterfly mode resonance characteristics are then anticipated in real-time in accordance with measured temperature variations. These parametric variations are translated in real-time utilizing a state space model to model deformation of the coil and carriage resulting from flexibility within the coil and carriage. A secondary displacement of the read/write head is thereby adaptively estimated in accordance with variations in physical plant parameters such as temperature utilizing the state space model. In response to the displacement estimation, the actuator control signal is dynamically adjusted to compensate for the computed secondary head velocity and displacement, thereby improving head positioning accuracy and increasing servo bandwidth.